Comparing arrays

Hello,

I advance in my postgres exploration and found something that
looks quite strange, or at least unexpected regarding the present
7.3.3 documentation.

In two table I store "objects" and their attributes. The
attributes, which are not meant to be searched, are stored as unbound
arrays of varchars. I have a query that needs to use those attributes
on both sides of an EXCEPT statement:

SELECT left.id, left.attribs FROM left
EXCEPT SELECT right.id, right.attribs FROM right;

That query can't be planed because of the following error: "Unable to
identify an ordering operator '<' for type 'character varying[]'".

I thought that I could build such an operator using PL/pgSQL,
unfortunately this language can't receive arguments of type ANYARRAY.
So this led me to the creation of a new ATTRIBUTES data type, the
should be acceptable as an argument to a PL/pgSQL procedure.

When I tried to create such a datatype, using a query modelled
after the documentation examples:

CREATE TYPE attributes (INPUT=array_in, OUTPUT=array_out,
INTERNALLENGTH=VARIABLE, ELEMENT=VARCHAR);

I'm signaled that the array_out procedure is not defined: "ERROR:
TypeCreate: function array_out(attributes) does not exist". That error
sounds strange as the CREATE TYPE manual describes uniform array type
creation as illustrated above and that array_out() seems to exist as
shown bellow.

SELECT proname, oidvectortypes(proargtypes) FROM pg_proc
WHERE proname LIKE 'array_%';

proname | oidvectortypes
---------------------+----------------------------
array_dims | anyarray
array_eq | anyarray, anyarray
array_in | cstring, oid, integer
array_length_coerce | anyarray, integer, boolean
array_out | anyarray
(5 rows)

All of this leads to the unavoidable questions:

1/ What went wrong with the ATTRIBUTES datatype creation?
How to correctly create it using 7.3.x backends?

2/ There may be better paths than creating a new datatype and
the associated operators that would permit using unbound
uniform arrays on both sides of an EXCEPT statement.
What would be such paths?

Regards.

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